Solar Generators with Parallel-Connected Series Strings

In this frequently used arrangement for larger solar generators, the modules that are used to generate the requisite system voltage are first wired to series strings comprising nMS modules (see Figure 4.28). To achieve higher aggregate current, a number (nSP) of such strings are wired in series. Figure 4.31

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nMp modules connected in parallel

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Figure 4.30 Пмр parallel-connected modules with module fuses. Parallel-connected modules of the same type generate only low reverse current under local shading. In order to allow for reliable handling of a PV generator malfunction, however, only three or four modules should be directly parallel connected. Module fuses should be used for nMp > 4 scenarios, and if such groups are series connected the parallel-connected module groups should be bypassed via a large bypass diode that can conduct 1.25 times the aggregate short-circuit current of all modules

bypass diodes (unless integrated in module)

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3-4 3-4 3-4

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circuit breaker (two-pole or four-pole)

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circuit breaker (two-pole or four-pole)

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circuit breaker (two-pole or four-pole)

Figure 4.31 Solar generators with parallel-connected series strings that provide optimal protection against all possible anomalies. The various strings are decoupled from each other via string diodes so as to obviate reverse current in functioning string diodes. In the event of string diode failure (often induced by a short circuit), the reverse current is limited still further by the circuit breakers; each string can be decoupled from its circuit breaker by the generatorbusses and measured separately

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Figure 4.32 Solar generator with low-cost parallel-connected series strings (fuses, disconnect terminals and special PV plugs in lieu of circuit breakers). Local shading of individual module strings does not induce excessive reverse current. However, string fuses should be integrated for more than three to four parallel-connected strings, so as to allow for handling of catastrophic solar generator failure induced by module or partial-string short circuits, or grounding faults (see Figure 4.33)

displays an arrangement where optimal solar generator protection is realized to take account of all possible anomalies. As noted in Section 4.3.1.1, the string diodes shown here can be dispensed with.

Two separate fuses can be substituted for bipolar circuit breakers, or unipolar circuit breakers or fuses can be used – which of course should only be removed when the entire system is shut down. In such a case, however, as is illustrated in Figure 4.32, a disconnect terminal should be installed on the opposite side of the string, or a special disconnect-enabled PV plug should be used (as indicated).

The wiring configuration shown in Figure 4.31 also makes it easier to monitor system operation by allowing for current measurement in individual strings. In the event of a string anomaly such as excessively low current, the failed string can be simply disconnected from the generator busses using the circuit breakers, whereupon the anomaly in the failed string can be localized and corrected without interfering with operation of the intact array. Hence this arrangement (or the simpler version in Figure 4.32 involving fuses and disconnect terminals) is by far the most prevalent modality for medium to large PV plants. However, the simpler connection arrangement shown in Figure 4.32 provides only suboptimal protection for awkwardly located ground connections. To obtain optimal protection in such a case, bilateral fuses should be used for the positive and negative conductors in each string.

Before solar generators with parallel-connected series strings are hooked up, one should verify that the various string voltages are approximately the same, so as to avoid needless mismatch power loss, or reverse current in the event of major wiring errors.

Cast bridge rectifiers are highly suitable for systems that integrate string diodes, whereby the following elements should be used: two or three low-cost insulated string diodes with a shared cathode in a well – insulated casing that readily cool down by virtue of being mounted on a metal plate.

When modules wired in series are shaded, their power drops instantaneously. However, the greater the number of modules in a series string, the lower the shading-induced power in the individual modules. Owing to the higher system voltage that comes into play here, a sizeable portion of the current in the
non-shaded modules will flow into the shaded module via the bypass diodes. Such mismatch losses can be mitigated by measuring the individual modules and only series-connecting modules in a string that present similar IMPP current levels at the MPP (for more on mismatch loss see Section 4.4).

Updated: August 5, 2015 — 12:26 pm